1. Field of the Invention
This invention relates to a method for purifying a polysilane containing organic solvent-insoluble matter and microgel.
2. Prior Art
Most often, polysilanes are synthesized by coupling reaction of polyfunctional halosilanes with alkali metals. Most polymer products resulting from this reaction contain organic solvent-insoluble matter and microgel as impurities. Particularly when the starting reactant is a trifunctional or polyfunctional halosilane, a large amount of insoluble matter forms. Additionally, insoluble matter forms in the polysilane during long-term storage.
In the process of preparing polysilanes, the resulting reaction product must be washed with water to remove the alkali halide by-product. The insoluble matter, if present in the reaction product, makes it difficult to separate the aqueous layer. Then the insoluble matter becomes the cause of low yields and low production efficiency.
On the other hand, when polysilanes are applied to photoresists, photosensitive elements, and organic devices, it is desired to form a uniform smooth film from a polymer solution. From the standpoints of yield, production efficiency and film formation, it is necessary to remove the insoluble matter and microgel from the reaction products.
One prior art practice for removing the insoluble matter is by dissolving polysilane in a good organic solvent and subjecting the polysilane solution to centrifugation or filtration. Since the insoluble matter cannot be completely removed by centrifugation, the centrifuged polysilane is unlikely to form a smooth film and thus unsuitable to apply to photoresists and analogues. The filtration method using paper and other filters can remove insolubles of a relatively large size, but not insolubles of a relatively small size, that is, microgel, resulting in a faintly turbid polymer solution. The addition of filter aids would occur to the skilled artisan, but fails to assist in the microgel removal. Polysilane films prepared from such a faintly turbid solution suffer from several drawbacks including low transparency and poor surface smoothness.
When membrane filters are used, clogging occurs because the insoluble matter and microgel have been swollen with the solvent. Then the separation of the insoluble matter and microgel by filtration becomes quite difficult or substantially impossible. A little improvement is achieved by adding filter aids although a very long filtration time is still necessary. This technique is not regarded practical.
It is thus desired to have an effective method capable of purifying a polysilane without the above-mentioned problems.